Polyvinyl alcohol (PVOH) is a water soluble, biodegradable resin with excellent aroma and oxygen barrier properties and resistance to most organic solvents. Other desirable attributes of PVOH include anti-static properties, grease and oil resistance and heat sealability. The polymer is used extensively in adhesives, textile sizing and paper coatings. However, the end uses of PVOH have been limited despite its excellent mechanical. physical and chemical properties to those uses in which it is supplied as a solution in water. This limitation is partly due to the fact that vinyl alcohol polymers in the unplasticized state have a high degree of crystallinity and show little or no thermoplasticity before the occurrence of decomposition which starts at about 170.degree. and becomes pronounced at 200.degree. C. below its crystalline melting point.
To a limited extent, this shortcoming has been overcome through the use of external or internal plasticizers. Regarding the use of external plasticizers, a common plasticizer for PVOH is blended into the polymer using a high intensity mixer and then the plasticized resin is fed into an extruder for melting and shaping of the formed article. Common external plasticizers include hydroxylic compounds, such as ethylene glycol, polyethylene glycol, neopentyl glycol, glycerine and non-hydroxylic compounds such as aromatic sulfonamides and aromatic phosphates as disclosed in U.S. Pat. No. 4,536,532.
Internal plasticization of PVOH is achieved with a comonomer through copolymerization, grafting or post reaction to breakup the crystallinity and to reduce the crystalline melt temperature below its decomposition temperature. For example, hydroxyethylation of PVOH is disclosed in U.S. Pat. Nos. 2,990,398; 1,971,662 and 2,844,570 and copolymers of PVOH and poly(alkyleneoxy) acrylates are disclosed in U.S. Pat. No. 4,618,648.
The desirable physical and mechanical properties of PVOH combined with thermoplasticity have opened a new market opportunity for PVOH, especially in packaging applications where the contents must be dissolved or dispersed in water. Examples of such packaging applications for these PVOH materials include pesticides which are applied as a water spray, caustic cleaners or detergents which are dissolved during use, and process chemicals such as pigments, dyes or carbon black which are dissolved or dispersed in water. The advantages of using a water soluble film for packaging the above materials include the following:
elimination of human exposure to highly toxic or concentrated chemicals; PA0 improvement in measuring accuracy; and PA0 elimination of the need to clean and discard toxic chemical containers after use. PA0 (a) 5 to 95 wt % PVOH, PA0 (b) 5 to 95 wt % thermoplastic PUR, and PA0 (c) 1 to 30 wt % plasticizer, based on polyvinyl alcohol. PA0 Greatest Shear--Least Forward Conveying Efficiency PA0 Least Shear--Most Forward Conveying Efficiency
While there are water soluble PVOH films available for packaging these materials, they typically have at least one of the following deficiencies:
Commercial PVOH films use low molecular weight PVOH for rapid cold water solubility. The low molecular weight makes the polymer undesirable for packaging one to ten pound quantities of pulverulent materials because the tensile strength. elongation and tear resistance are poor.
Low temperature impact strength is poor if 3 to 10% water is used as a coplasticizer, since water loses its plasticization effect below 0.degree. C. causing the PVOH film packages to embrittle at low temperature. This limits the uses of PVOH film in cold climates.
The physical properties of this hygroscopic polymer such as tensile strength and elongation are dependent on atmospheric humidity. Therefore, it elongates and blocks at high humidity, whereas at low humidity it is a stiff and low enlongating film. This reduces the shelf stability of the film and could limit its end uses in dry environments.
U.S. Pat. No. 4,464,438 discloses a composition comprising three essential components, namely, PVOH, copoly(ethylene-vinyl alcohol) and a polymeric processing aid, such as a polyamide or polyurethane, and having excellent grease barrier properties.
U.S. Pat. No. 4,611,019 discloses a blend of selected polyamides or polyesters to plasticize melt extrudable PVOH for improved oxygen barrier properties.